1. Field of the Invention
This invention relates, in general, to digital information storage systems, and in particular, to high capacity hard disk drives.
2. Description of Related Art
Digital data may be stored on flexible or "floppy" disks or on hard or Winchester-type disks by the magnetization of successive small areas on the magnetic surface of the disk, by means of a magnetic head or "slider", as the disk rotates. The density of digital storage on hard disk memory systems is on the order of 10 to 20 times the density achieved with floppy disk memory systems.
In the field of hard disk systems, sometimes referred to as Winchester-type disk systems, the rigid disks are normally formed of an aluminum alloy, and have a magnetizable coating on their upper and lower surfaces.
Rigid magnetic storage disks were originally relatively large in diameter, but in the last few years, the size has been reduced so that 51/4 inch disk systems are now widely available. Further, industry standard dimensions have been established for 51/4 inch Winchester-type disk units, and these dimensions include a height of 3.25 inches (82.6 millimeters), a width of 5.75 inches (146 millimeters), and a depth of 8.00 inches (203 millimeters). Environmental standards have also been established, with the operating temperature extending from 10.degree. C. to 50.degree. C. and the non-operating or storage temperature extending from -40.degree. C. to +65.degree. C.
By using a flat type of drive motor, it has previously been possible to mount five hard disks of the 51/4 inch size within the industry standard dimensions mentioned hereinabove. This type of configuration would provide a storage capacity on the order of 170 megabytes.
However, increasingly sophisticated computer programs, coupled with increasingly complex processors have created an accelerating demand for additional rapid access storage, and accordingly, there is a demand for 51/4 inch disk drives with larger capacities. One way of increasing the number of disks which may be included within the industry standard dimensions is to locate the drive motor centrally with respect to the storage disks, with the disks being directly secured to the motor rotor. However, the rotor involves magnetic material, usually steel, and the storage disks are normally made of aluminum which is then coated on both sides with a magnetizable material. Unfortunately, the thermal coeffiecient of expansion of iron or steel is on the order of 6 to 9 microinches per inch per degree Fahrenheit, whereas the thermal coefficient of expansion of aluminum is on the order of 11-13 microinches per inch per degree Fahrenheit. Thus, if the aluminum disks were to be rigidly mounted to an outer steel cylinder forming part of the motor rotor or bearing periphery, the difference in thermal expansion over the range of temperatures through which the disk drive is to be operative, would cause the disks to flex or deform, so that the precision digital memory would not operate reliably. In this regard, it is to be noted that one of the disk surfaces is normally reserved for servo-control, so that normal and uniform expansion and contraction of the aluminum disks, without deformation or buckling, causes no problem. However, if the disks flex, or depart from their normal flat surface configuration, reliability will suffer severely, or the units may become wholly inoperative for digital storage.
A method of construction of a hard disk assembly which overcomes the thermal coexpansion problem by utilizing a thermal shrink fit engagement between the cylindrical steel member and the aluminum hard disk mounting assembly is described in my co-pending application entitled, "High Capacity Winchester Disk Drive", Ser. No. 911,637, filed on Sept. 25, 1986, now U.S. Pat. No. 4,717,977, and assigned to the same assignee as the instant invention.
Accordingly, a principal object of the present invention is to provide an alternative method for overcoming the problems of differential thermal expansion when aluminum memory disks are employed with a central motor having a steel frame or outer cylindrical portion of the rotor, as outlined hereinabove.